This invention relates to a magnetic-field detector including a magnetic-field-to-voltage transducer, such as a Hall element or magneto-resistor bridge, followed by a comparator for producing a high binary output signal when the ambient magnetic field exceeds a predetermined level; and more particularly pertains to a chopped low power magnetic-field detector having hysteresis memory.
Magnetic-field detectors for use as a compass, including a Hall element as the magnetic-field-to-voltage transducer, are described in the U.S. Pat. No. 3,197,880 issued Aug. 3, 1965 and in U.S. Pat. No. 4,163,326 issued Aug. 7, 1979. These detectors provide sinusoidal driving currents for energizing non-co-planar multiple Hall elements and produce an analog signal (that may be converted to a digital signal) indicating compass heading, but do not chop a Hall element on and off and have no need for a hysteresis feature. An electronic compass, employing a Hall elements array is described in the U.S. Pat. No. 5,241,270 issued Aug. 31, 1993, wherein the polarity of the summed outputs of Hall elements is determined by a largest voltage sensing circuit and that largest voltage is converted to digital form and held in a buffer that is connected to a digital display, indicating compass heading.
It is well known to make latching magnetic-article-proximity detectors comprising a Hall latch having a Hall element a Hall voltage amplifier, and a Schmitt trigger circuit or other latch with hysteresis for producing a binary output signal. Examples of such latches are described in the U.S. Pat. No. 3,816,766 issued Jun. 11, 1974 to Anselmo et al, U.S. Pat No. 4,443,716 issued Apr. 17, 1984 to G. Avery and U.S. Pat. No. 4,705,964 issued Nov. 10, 1987 to J. Higgs. These patents are assigned to the same assignee as is the present invention.
Such a latching magnetic-article-proximity detector having a temperature compensation feature is described in U.S. Pat. No. 4,908,527 issued Mar. 13, 1990 to J. C. Van Antwerp, wherein the Schmitt-trigger hysteresis function is achieved a zero-crossing comparator having a switched feedback circuit connected from the comparator output to input. The amount of feedback depends upon a pair of feedback resistor values and a current source that depends upon a reference resistor value. By making these three resistors of predetermined temperature coefficients, the operate and release points of the composite Schmitt trigger circuit are temperature compensated with respect to changes in magnet strength, Hall sensitivity and other parameters with temperature.
In the U.S. Pat. No. 4,283,643 issued Aug. 11, 1981 to H. P. Levin, there is described an electronic Hall sensor for generating a signal current having a fixed phase relationship with the current in a power distribution conductor. The energizing current in the Hall element is activated and deactivated in response to command signals in order to limit errors due to temperature drift attributed to the flow of Hall energizing current.
It is an object of this invention to provide a latching magnetic-field detector in which the Hall element is chopped to reduce the energy drawn from the power supply, there being no loss of hysteresis memory attributable to the chopped Hall-element energizing current being turned off for a high percentage of each chop cycle.